Under-the-Skin ICD Affirmed Safe, Effective

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The subcutaneous ICD was safe and effective based on full trial results, although the device still has limitations.

Note that the results of the present study suggest that the subcutaneous ICD is a viable alternative to transvenous systems among patients who do not require pacing therapy for heart failure, bradycardia, or ventricular tachycardia.

The subcutaneous implantable cardioverter-defibrillator (ICD) was safe and effective in the full pivotal trial results finally being published, but the device still has limitations.

The S-ICD System met both its primary trial endpoints with a 180-day complication-free rate of 99% and induced acute ventricular fibrillation conversion rate of 100% in evaluable patients, Martin C. Burke, DO, of the University of Chicago Heart Rhythm Center, and colleagues found.

"The results of the present study indicate that the subcutaneous ICD is a viable alternative to transvenous systems among patients who do not require pacing therapy for heart failure, bradycardia, or ventricular tachycardia," they concluded online in Circulation: Journal of the American Heart Association.

In an editorial accompanying the published results, Leslie A. Saxon, MD, of the University of Southern California Los Angeles, called them reassuring but agreed on the limitations.

"Although these data are reassuring and comparable to transvenous ICD success rates, the overall number of treated episodes is incredibly small in comparison with the data on transvenous defibrillator therapies delivered outside the hospital, over the life of the device, that are available for analysis in tens of thousands of patients," she wrote.

She also cited limitations to the technology, such as lack of remote interrogation capability, that may limit how widely it can be used in clinical practice.

In deciding between a transvenous device and a subcutaneous ICD in practice, she suggested the following: "The expectation of the need (assuming a battery longevity of 5 to 8 years for the subcutaneous device) for bradycardia pacing, a left ventricular lead for biventricular pacing, or the need for antitachycardia pacing argues for the transvenous device."

"Patients with advanced symptom class heart failure or very depressed ventricular function may face additional risk at implant because of the need for at least two ventricular fibrillation inductions and the longer times to defibrillation associated with the subcutaneous device," she added.

The S-ICD System offers an 80-joules shock for ventricular arrhythmia defibrillation with ventricular bradycardia pacing for 30 seconds afterward at 50 bpm, but no cardiac pacing beyond that or dual chamber sensing.

The pared-down device eliminates the need for vascular leads and their accompanying risks of infection, failure, and difficult removal, and there are still a significant minority of patients for whom it would fit the bill, Saxon noted.

"These predominately include those with prohibitive vascular access issues and those at heightened risk for major systemic infection with an indwelling chronic vascular lead," she noted.

The study did offer some useful details about performance of the system but still left some questions unanswered, Saxon noted.

For example, it wasn't clear whether the study adequately answered the issue of acute defibrillation testing the device requires, which can be associated with patient instability and postoperative complication.

No adverse patient outcomes were associated with defibrillation testing in the rest, and all were able to achieve two consecutive defibrillation at 65 joules.

But 16 of the 314 successfully implanted patients were excluded from acute defibrillation testing, "mostly because of implanting physician concerns regarding the risk of rendering the patient clinically unstable."

And 17% of the patients required shocking wave polarity reversal to achieve those two successful conversions, "increasing the number of ventricular fibrillation inductions required during implant."

Overall in the 11 months of follow-up, the device terminated spontaneously-occurring ventricular tachycardia or fibrillation in 6.7% of the patients implanted (21 of 314).

However, all of these patients successfully converted, predominantly on the first or second shock (92% and 97%, respectively), and two patients had multiple successful shocks for ventricular tachycardia storm.

The primary effectiveness endpoint excluded the 16 patients who didn't complete the full testing protocol and one another patient who didn't undergo any testing because of persistent left ventricular thrombus.

But the researchers pointed to a sensitivity analysis in which counting all those cases as failures still left the acute ventricular fibrillation conversion rate at 95%, with a lower bound for the 95% confidence interval of 92%, exceeding the performance goal of 88%.

The 180-day rate free of system complications was 99%, with a lower 95% CI of 98%, well above the performance goal of 79%.

Four infections requiring explantation occurred, all early in the trial; none of the 13 patients with a superficial or incisional infection required chronic antibiotics. No ICD-related endocarditis occurred.

Inappropriate shocks occurred in 13.1% of patients, which Saxon noted was similar to transvenous ICDs.

However, she pointed out that without remote monitoring capability, it's hard to track inappropriate shocks caused by oversensing versus atrial fibrillation with a rapid ventricular response.

"This is concerning, because the population enrolled in the subcutaneous ICD study were 10 to 20 years younger than the standard transvenous ICD recipient," Saxon pointed out. "The age of the subcutaneous ICD recipients indicates that they may be a more active and more prone to oversensing owing to subcutaneous sensing challenges or T-wave double counting."

Programming an arrhythmia discrimination zone at discharge cut the oversensing-related inappropriate shocks by a relative 56% from 14% to 6%. It cut supraventricular tachyarrhythmia-related inappropriate shocks by a relative 70% from 10% to 3%.

Study limitations included lack of a control group and short duration of follow-up.

The S-ICD System Investigational Device Exemption Clinical Study was sponsored entirely by Cameron Health, a subsidiary of Boston Scientific.

Burke and several co-authors reported being consultants for, and having received honoraria and research support, from Cameron Health and from Boston Scientific.

Saxon reported being on the medical advisory board of Boston Scientific, having received research funds from Boston Scientific, Medtronic, and St. Jude Medical, and being a principal investigator at her center for the S-ICD System Safety and Effectiveness Study.

Reviewed by Robert Jasmer, MD Associate Clinical Professor of Medicine, University of California, San Francisco and Dorothy Caputo, MA, BSN, RN, Nurse Planner

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